Was there only one actor that played Daniel Jackson in sg1?

No. There were two actors who played Daniel Jackson in SG-1. James Spader played Daniel Jackson in the 1994 feature film Stargate, and Michael Shanks played Daniel Jackson in the SG-1 television series and the direct-to-video films Stargate: The Ark of Truth (2008) and Stargate: Continuum (2008).